Q&A: How Rain Screens Work

Asphalt Shingle Nailing

Q.I plan to install wood
siding over a vented rain screen on a house in Seattle,
where we get rain for long periods during the winter.
My question is, how can the air cavity dry if the
exterior relative humidity is 100 percent? Assuming the
air cavity temperature is the same as the exterior
temperature, the air coming through the cavity
can’t pick up any additional
moisture.

A.Corresponding
editor Paul Fisette responds: If the relative
humidity is in fact 100 percent, then of course
there will be no drying. However, since the
relative humidity is not 100 percent all the time,
some drying will occur during these
periods.

Remember, the rain screen provides you with
insurance. The siding keeps most of the liquid
water out of your wall system, and the screen
provides backup protection. The air space allows
any water that gets past the siding to drain, and
the circulating air helps dry the back of the
siding and the wall when they get wet.